Holy looming planets, Batman! It has already been observed that Mike Cahill's Another Earth and Lars Von Trier's Melancholia share the evocative image of another heavenly body in close proximity to Earth. For me, though, the most significant thing about this coincidence is that neither film would normally be classified as science fiction. And it's not as if either director is distancing himself from the term, the way Margaret Atwood seems to be. She's made increasingly baroque contortions to explain that what she writes is "speculative fiction" and not SF. (Though surely the term "speculative fiction" is tautological. Isn't all fiction speculative?)

There has long been a tendency for SF themes to bleed into the mainstream and non-SF genres. What is It's a Wonderful Life if not a story set in a parallel universe? And we've come to expect a touch of SF in our action films; as far back as the 1960s, James Bond and other spy movies flirted with technology so farfetched it tipped over into futuristic, and it's rare for a Hollywood thriller now to pay much heed to the laws of physics. But more and more high-concept, big-budget action flicks – Limitless, The Adjustment Bureau, In Time are just three recent examples – are coming out of the closet as unabashedly SF, even though not one of them features what Atwood refers to as "talking squids in outer space".

These films are aimed at audiences who probably wouldn't object to talking squids, but I've written before about the way chronological jiggling, time warps and parallel universes have infiltrated mainstream drama, romcoms and sitcoms. And "SF creep" into the mainstream and arthouse is on the rise, even if the term "science fiction" is only mentioned by critics disparagingly, as if the fact that the film under question refuses to classify itself as that makes it superior to the usual genre nonsense.

Take three other 2011 high-profile releases. Never Let Me Go was sold as Brit-lit (tagline: "Based on the best selling novel"), whereas it was really The Island for People Who Don't Like Explosions, with Keira Knightley instead of Scarlett Johansson. Hanna was sold as a junior Bourne Identity ("Adapt or die"), whereas it was essentially a junior Universal Soldier ("Robots run amok"). Midnight in Paris was basically a time travel yarn, but was sold as a Woody Allen film, which is permissible since he's a genre unto himself and has already flirted with SF in Sleeper, Everything You Always Wanted to Know About Sex … and – in the most memorable bit of Stardust Memories – aliens. ("We enjoy your movies. Particularly the early, funny ones.")

It's tempting to dismiss such films as SF for People Who Don't Like Science Fiction, since mainstream pontifications on the genre still yield blinkered pronouncements from folk who wouldn't be caught dead at a Star Trek movie. Conversely, a lot of people who might have enjoyed Monsters were probably put off by it being marketed as a variation on District 9 ("After six years, they're no longer aliens. They're residents") when it was really the sort of thing that plays better at Sundance than at a fantasy festival – a low-budget relationship movie that happened to have tentacled aliens as walk-on extras.

But it's more likely a symptom of the way boundaries between traditional genres are dissolving. So maybe it's time to redefine genres, or even nominate new ones. Since barely a week goes by without another set of actors doing give-me-my-Oscar-now acting in overwrought drama triggered by a traffic accident (Another Earth, Margaret, Rabbit Hole etc) I propose "roadkill" as a new genre. Remakes, sequels and prequels can be lumped together under "recycling". Then there's the "girls in fetishwear" genre typified by Sucker Punch and Colombiana. This Year's Leslie Mann Movie would take care of all those bromances involving her husband, Judd Apatow. And we mustn't forget The Tom Cruise Running Very Fast film.